Split the docs generation into a separate component

[avdv]

Is your feature request related to a problem? Please describe.

Currently, every component depends on java, posix and the cc toolchain (even the core). This is because of the stardoc dependency which consists of a java_binary and is used to generate documentation from starlark.

Ideally, the core should not depend on any other toolchain at all. Other toolchains should also not depend on any other toolchain that is indeed not used by it.

Describe the solution you'd like

Make the docs generation a component of its own.

Describe alternatives you've considered